This invention relates to a vacuum interrupter contact comprising a metal coated layer on a contact substrate and a process for producing the same.
FIG. 6 shows a sectional view of a vacuum interrupter (also called as vacuum circuit breaker or vacuum valve). The vacuum interrupter has a substantially cylindrical insulating vessel 20, and a vacuum vessel constituted of end plates 23, 24 made of a metal mounted through sealing fittings 21, 22 at both end faces of the vessel 20. Internally of the vacuum vessel is formed a shielding chamber 25 with a vacuum atmosphere.
Within the shielding chamber 25, there are arranged a first electroconductive rod 26 mounted fixedly extending through the end plate 23, and a second electroconductive rod 27 mounted movably in the axial direction extending through the end plate 24. At the respective opposed ends of these first and second electroconductive rods 26, 27 are mounted a movable electrode 29 having a contact 28 and a movable electrode 31 having a contact 30 so as to be opposed to each other.
Between the second electroconductive rod 27 and the end plate 24, there is mounted a bellows 34 for maintaining air tightness. Outside of the bellows is mounted a bellows cover 35 for protecting the bellows 34 from the arc vapor generated between both contacts 28, 30. Further, internally of the vacuum vessel, there is provided an arc shield 36 for protecting the insulating vessel 20 from the arc vapor. The vacuum interrupter is adapted to be opened and closed according to a driving mechanism (not shown) through the electroconductive rod 27.
Next a description is provided by referring to FIG. 7. On the electroconductive rod 27, a movable electrode 31 is mounted by a soldering portion 32. On the movable electrode 31 is mounted a contact through the soldering portion 33. The contact 30, after having been worked into a predetermined shape, is assembled into the shielding chamber soldered onto the electrode or the electroconductive rod. If necessary, a conditioning treatment is applied to clean and finish the surface. Also, as similar materials for contact, alloys such as Cu-Bi type, Cu-Te type, Cu-W type, Cu-WC type, Cu-Cr type, Cu-Ti type, etc. may be used depending on the intended.
It is demanded for this kind of vacuum interrupter contact that its surface should not be contaminated with oxide coating, etc. However, the contact material has the problem that surface oxidation proceeds in the course from working to assembling in the vacuum interrupter. Besides, depending on the management situation during that time, there are the following problems in that the state of contamination is not constant and the quality is not stabilized.
First, such contamination cannot be removed easily even by conditioning treatment, and therefore there is the problem that the contact resistance characteristics becomes unstable.
To cope with this problem, the practice has been to attach a metal coating such as Cu onto the surface of the contact substrate, thereby alleviating any influence from atmosphere. In the case of this countermeasure, for stabilizing contact resistance, the metal coating is required to be formed sufficiently thick. However, if it is too thick, not only lowering in welding resistance as the contact is brought about, but also the adhesion strength between the coating and the contact substrate is weakened to give rise to peeling, whereby there is also a problem that lowering in dielectric strength characteristics may be brought about.
In the case where the cause for of making the contact resistance higher resides in formation of an oxide coating of an active metal such as Al, Ti, Cr, etc., it may be considered to remove mechanically the oxide coating by blasting of rigid particles. However, in this case, the blasted rigid particles may penetrate into the matrix to remain there, and also the surface roughening proceeeds, whereby there is the problem that no necessarily satisfactory value in dielectric strength characteristics can be obtained. Also, for a thermally stable coating such as on oxide of Al, Ti or Cr, time and expenses are consuming for carrying out reduction with hydrogen or voltage-current conditioning treatment which is inconvenient without obtaining sufficient effects in many cases.
The next problem is that, when the vacuum interrupter contact is generally solder bonded to an electrode in a hydrogen reducing atmosphere, or nitrogen or vacuum atmosphere, the strength of the solder bonded portion is affected by contamination of the oxide coating of the vacuum interrupter contact, and depending on the state of such contamination, the strength at the solder bonded portion may become extremely weak. In the contact material containing an active metal such as Al, Ti or Cr, there is the problem that formation of an oxide coating exerts an influence on the strength of the solder bonded portion.